Work is continuing on the structure and activity of diphtheria toxin. Studies of the heterogeneity of diphtheria toxin, particularly with respect to binding and hydrolysis of NAD, are being pursued, as are attempts to synthesize a toxin of hybrid specificity by cross linking Fragment A to selected receptor-binding proteins or protomers. Attempts to define conditions for growing x-ray grade crystals of Fragment A will be made. Studies are also being carried out on exotoxin A from Pseudomonas aeruginosa and cholera toxin. A new plate assay for mutants in cholera toxin production is being used to obtain a collection of over-and under-producers, which will be used for genetic and biochemical analysis. Efforts are being made to study the enzymic and cell-binding properties of exotoxin A in more detail. BIBLIOGRAPHIC REFERENCES: Mekalanos, J.J., R.J. Collier, and W.R. Romig. (1977) A simple method for purifying choleragenoid, the natural toxoid of Vibrio cholerae. Infection and Immunity, in press. Chung, D.W., and R.J. Collier. (1977) An enzymically active peptide from the ADP-ribosylating toxin of Pseudomonas aeruginosa. Infection and Immunity, in press.